Research conducted by Southern Methodist University in Dallas, Texas found that husbands and wives who are satisfied with their marriages gained weight over a period of four years, compared to those in unhappy relationships who didn’t gain weight. One possible reason: People who believed they had found their true love were more comfortable with their bodies than they were when trying to first attract a mate.

“When you first fall in love, the brain is flooded with dopamine, a stimulating chemical that suppresses the appetite,” Helen Fisher, PhD., a biological anthropologist at Rutgers University, tells Yahoo Beauty. “When people experience a rush of dopamine, they can’t sleep at night because they’re thinking about the other person, they fixate over whether that person will text them back, and they feel optimistic and sexually possessive.” One study even found that being in love activates the same portion of the brain associated with addiction, which helps explain why you feel obsessed with the object of your desire.

According to Fisher, people can experience this blissful “brain system” called romantic love, for anywhere from one to five years. “We’ve even scanned the brains of couples married for 25 years who swore they were still madly in love,” says Fisher. “For example, if you’ve been with your partner for many years but only see them periodically, it’s possible to still feel passionately in love,” she adds.

The next is called attachment, during which people fixate solely on their partners. “There’s a sense of calm and comfort and it’s common to feel at times as though you both love and hate that person,” says Fisher. And here’s where weight gain often occurs: If you’re spending lots of time together, you’re likely cooking and eating the same meals, and feeling more confident in your feelings for each other. As a result, you may ease up on the fitness and health habits you worked hard to maintain at the beginning of your relationship.

From an evolutionary perspective, feeling bonded to one mate allowed couples to successfully raise their offspring after their lustful feelings died down. And if you’ve gotten this far in a relationship, unhealthy habits will be even harder to break.

“However, it’s important to note that not all couples experience the brain systems in one particular order,” says Fisher. “Relationships can begin at any stage and fluctuate depending on variety of factors. In a good relationship, you want to keep all systems alive.”

Of course, this doesn’t mean that long-term couples can’t stay fit. Not only will hitting the gym together keep you healthy, physical exercise is scientifically proven to boost your sex drive. #Relationshipgoals.